The present invention relates to electric heating devices in general and in particular to heating elements useful in such devices.
Heating devices employed for household, industrial or other use have typically a heating element which is embedded within the device and which then transfers heat by radiation, convection or conductance to an output surface of the device. The temperature at the output surface of the device is thus much lower than the working temperature of the heating element. There is usually a very big temperature drop between the temperature of the heating element which ranges from 100""s to 1,000""sxc2x0 C., depending on the type of device, to a temperature at the output surface which may range from 60-90xc2x0 C. for household heating devices to 120-300xc2x0 C. for household cooking and baking devices.
Heating devices come in a large variety of form and shape. For example, one type of electric heating device has a bare heating element typically fashioned as either a band or a wire made from an alloy containing nickel and/or chromium and which typically reaches working temperatures ranging from about 400xc2x0 C. to 1600xc2x0 C. The heat generated by this type of heating elements is dissipated to the surrounding medium by mainly one or any combination of three heat transfer mechanisms, these being radiation, natural or free convection or forced convection (e.g. by the use of a ventilator). Such heating devices enjoy the advantages of being inexpensive, small, of a relatively low weight and having a long lifetime. However, they suffer from a drawback arising out of the high working temperature of their heating elements which poses a safety hazard.
In another type of electric heating device, commonly known as the electric radiator, the hazards associated with the bare heating element type of heating device are eliminated by submerging a heating element in a reservoir of oil or a similar liquid employed for transferring the heat generated by a heating element to the external walls of the radiator. Typically, the output temperature of a domestic heating radiator is about 70xc2x0 C. whereas the working temperature of its heating element is 700xc2x0 C. or above. Consequently, such a domestic heating radiator is typically equipped with heat-insulating means. As is well known, electric radiators suffer from the disadvantages that they are expensive, heavy, and relatively inefficient.
U.S. Pat. No. 2,600,486 discloses an electric heating element which comprises a flexible conducting metal sheet in which slits are cut so as to form an elongated relatively narrow torturous flow path for an electric current. A similar kind of heating device is also disclosed in U.S. Pat. No. 3,584,198. U.S. Pat. Nos. 3,525,850 and 4,551,614 disclose an electric heater comprising elongated heating elements in the form of corrugated matallic ribbons which are heated to a temperature ranging from about 1200xc2x0 F. to 1800xc2x0 F. (about 650-1000xc2x0 C.). U.S. Pat. No. 2,719,213 discloses a heating device in the form of a flat panel which comprises an electric conductor arranged in a plane between two different non-conducting sheets or layers, and French Patent Specification 975,038 discloses a heating element in the form of arrogated plates. Another heating panel is disclosed in U.S. Pat. No. 3,244,858 wherein an electric heating wire is arranged in a plane to track a zig-zag path over both sides of a non-conducting planar core. U.S. Pat. No. 4,203,198 discloses a planar heating device employing a heating element arranged in a plane to track a tortorous path and sandwiched between two sheets of fiber glass. Another heating device with a heating element arranged in a plane between two insulating sheets is also known. U.S. Pat. No. 4,032,751 discloses a planar heating element utilizing electrically conducting carbonaccous pyropolymers. An electric planar heating device intended for use as an electrical bandage is described in U.S. Pat. No. 2,712,591, wherein an electrically conducting ribbon is embedded in a resilient strip of insulating material. A flexible circuit heater which can be used within clothing or the like is disclosed in U.S. Pat. No. 4,948,951, utilizing an electrically conductive strip made to track a torturous path within a flat flexible member. U.S. Pat. No. 4,665,308 discloses an electrical heating element that can be incorporated in the lining of an item of clothing which makes use of a ductile insulated metal wire fixed to a metal sheet.
There is accordingly a need in the art to provide an electric heating device for heating an object to a required temperature that eliminates the need for thermo-insulating means or at least substantially reduces the requirements to such means.
It is an object of the invention to provide a novel heating device with as low as desired temperature gradient between the temperature of the outer surface of a heating element and the required temperature of the heated body.
It is an object in accordance with some embodiments of the invention to provide a heating device wherein the heating element constitutes the heat dissipating, output surface of the device.
It is an object in accordance with some other embodiments of the invention to provide a heating device wherein the heating element is embedded in or forms a structural element of a household object serving also a purpose other than heating.
It is flrhermore an object of the invention to provide domestic enclosure heating devices, electric cooking devices and therapeutic heating devices having characteristics in accordance with the above objects.
It is furthermore an object of the invention to provide a method for designing in constructing such heating devices.
Other objects of the invention will be clarified after reading the text below.
Generally speaking, the present invention provides a method and device capable of a so-called xe2x80x9clow-gradientxe2x80x9d heating. In practice, in most cases it is desired to have substantially low gradient between the surface temperature of a heating device and the temperature of the heating object, so as to reduce the danger of fire, as well as the danger of bums, when applying a heating device to the body for medical purposes. Additionally, such a low-gradient heating enables to reduce the requirements of electro- and thermo-insulation, and to increase the effectiveness of heating (i.e., achieving a desired effect at minimal energy). This means that in order to create a high-quality heating device, its heating element, intended for heating a given object, should be independently designed and calculated, taking into account not only the power required to obtain a desired temperature of the object, but also the fact that the surface temperature of the heating element should be as low as desired different from the required temperature of the heated object. This is a very complicated task, since the transfer of heat from the heating element towards the object is proportional to the temperature gradient (i.e., to the difference between the temperature of the heating element and the temperature of the heated object). Accordingly, the desired decrease of the temperature gradient should be compensated, and practically, the single solution for this is a maximal increase in the surface area of the heating element.
Thus, the main idea of the present invention consists of designing a heating element whose surface temperature and surface area are independently selected at the given power required for heating a given object to a required temperature.
It was found by the inventor, that the solution for the above task is based on directionally varying the ratio between an electric current passing through the heating element and voltages at its input and output circuits. For the purpose of independent control of the surface temperature and surface area of the heating element, materials for the heating element should be selected in accordance with their physical parameters, namely, the specific resistivity and dimensions of the heating element, wherein the latter is in the form of a planar band having a substantially rectangular cross-section.
The inventor has found that in order to obtain the required effect, i.e., independent control of the surface temperature and surface area of the heating element at a given power, the width, b, and length, l, of the heating band should be selected so as to satisfy the following relations:       b    ≥          k      ·      I      ·                        ρ          δ                          l    ≥          k      ·      U      ·                        δ          ρ                    
wherein I and U are the electric current (A) and voltage (V) of the heating element; xcfx81 is the specific resistivity (Ohmxc2x7mm2/m); k is the so-called xe2x80x9ccorrection coefficientxe2x80x9d selected in accordance with the following two considerations: first, the value of k should be increased in those cases, when the surface area and the surface temperature of the heating element should be, respectively, increased and decreased; second, k should be of the same magnitude in the above equations, wherein the length units are meters, and the width and thickness units are millimeters.
The physical sense of the above equations will now be explained. The relationship between the heat power rating W of an electric heating element and its surface area S required to dissipate the heat generated thereby, can be approximated by the following equation:
S=k1W
Here, k1 is the coefficient that depends on a variety of factors, including the surface temperature of the heating element, the medium surrounding the heating element (e.g., air, water or another fluid), the ambient temperature of the surrounding medium, and to a lesser degree, the location of the heating device within its local environment, and the like. As such, the values of k1 can be obtained empirically by an artisan for various specific cases. For example, it is known that an electric heating element having a surface temperature of about 70-90xc2x0 C. requires about 0.7-0.8 m2 surface area to dissipate 0.5 Kwatt in an environment having an ambient temperature of 20xc2x0 C.
According to the invention, the heating element is a planar band having a length l along which the potential U falls, and a generally rectangular cross section having a width b and a thickness xcex4. As such, the heating element has one or two major surfaces (as the case may) each having an area lb, through which heat is dissipated. The surface area S of the heating element can be approximated by the following equation:
S=k2bl
wherein k2 is a coefficient which is approximately equal to 1 or to 2, depending on the number of surfaces through which heat is dissipated.
As is well known, the power rating W of an electric device, for example, an electric heater, is defined as the product of an operating potential U and an operating current I, i.e., W=IU. The operating potential U is equal to the product of the conductor""s operating current I and electrical resistance R, i.e., U=IR. Further, the electrical resistance R of a conductor depends on the specific resistivity of an electrically conductive material from which the conductor is fabricated and the physical dimensions of the conductor, in accordance with the following equation: R=xcfx81l/A. Here, xcfx81 is the specific resistivity of the electrically conductive material, l is the length of the conductor, and A is the cross-sectional area of the conductor, i.e., A=bxcex4.
Thus, from the above equations, we have:
W=I2R;W=U2/R
S=k1I2R;S=k1U2/R;S=k1I2xcfx81l/bxcex4S=k1U2bxcex4/xcfx81l
Hence,
k2bl=k1I2xcfx81l/bxcex4k2bl=k1U2bxcex4/xcfx81l
The length l and width b of the heating element can now be expressed as follows:       l    =                                                      k              1                                      k              2                                      ·        U            ⁢                        δ          ρ                          b    =                                                      k              1                                      k              2                                      ·        I            ⁢                        ρ          δ                    
It is thus understood that k=(k1/k2)xc2xd.
To more clearly illustrate the essential features of the present invention, let us consider the following example of designing a heating element of a heating device intended for use in physiotherapy. Assume that the surface temperature of the heating element has to be in the range of 41-43xc2x0 C. From experimental results, it appears that the coefficient k equal to 2 corresponds to this temperature value. A band of stainless steel with the thickness xcex4 of 0.05 mm and specific resistivity xcfx81 of 0.7 Ohmxc2x7mm2/m is selected as an electrically conductive material for fabricating the heating band. It is known that, in this specific application (i.e., physiotherapy), the power to be transferred to an object to be heated (i.e., a patient""s body) is about 50 Watt. Let us consider two possible cases: (1) The selected voltage U is 10V, and consequently, the electric current I is 5A; and (2) the voltage U and current I are, respectively 5 V and 10A.
Hence, for the first case, we have:       b    1    =                    2        ·        5        ·                              0.7            0.05                              ≈              37.4        ⁢                  xe2x80x83                ⁢        mm        ⁢                  xe2x80x83                ⁢                  l          1                      =                  2        ·        10        ·                              0.05            0.7                              =              5.4        ⁢                  xe2x80x83                ⁢        m            
The surface area S1 of the heating element through which heat is dissipated is calculated as follows:
S1=b1xc2x7l1=37.4 mmxc2x75.4 m=2xc2x7105 mm2
For the second case we have:       b    2    =                    2        ·        10        ·                              0.7            0.05                              ≈              74.8        ⁢                  xe2x80x83                ⁢        mm        ⁢                  xe2x80x83                ⁢                  l          2                      =                  2        ·        5        ·                              0.05            0.7                              ≈              2.7        ⁢                  xe2x80x83                ⁢        m            
The surface area S2 of the heating element through which heat is dissipated is calculated as follows:
S2=b2xc2x7l2=2xc2x7105 mm2
It is thus evident that, the variation of the ratio between the electric current I and voltage U, with the given constant power and with the constant surface area of the heating element (i.e., with the fixed temperature), enables to control the geometry of the heating element, namely, to control the ratio between the length and width of the heating element.
If the temperature of the heating element has to be changed with the same power, it can be achieved by selecting an appropriate value of the coefficient k, keeping in mind that an increase in the value of k leads to an increase in the surface area S, and, consequently, leads to a decrease in the surface temperature of the heating element.
The ratio between the length and width of the heating element can be affected by the selection of an appropriate electrically conductive material. As follows from the above equations, under the fixed values of current and voltage, the higher the specific resistivity xcfx81 of the heating element and the lower its thickness xcex4, the higher the length l of the heating element and the lower its width b.
Thus, the present invention allows for simultaneously and independently controlling the temperature and geometry of a heating element. This is a new approach, which has never been used for designing heating elements. According to this approach, initially, parameters defining the requirements of a specific application are set, namely, the power for heating a given object to a required temperature, and the voltage to be supplied to the heating element in accordance with a predetermined application of the heating device. It should be understood that these parameters, i.e. power and voltage, are defined independently. Then, the surface area of a heating element through which this power is to be dissipated is determined, and the physical parameters of the heating element (length, width, thickness and specific resistivity) are selected to meet the requirements of the specific application.
The present invention relates to domestic, industrial and other heating devices suitable for a wide range of applications including, but not limited to, heating enclosed spaces, heating food, therapeutic purposes and the like, and relates to electric heating elements for use therewith.
In the following, the term xe2x80x9cheating devicexe2x80x9d will be used at times. This term should be construed in a broad manner mainly to relate to any device or object wherein one or its intended uses is heating. In accordance with the prior art, a heating device is typically a dedicated device designed for a single function, namely heating. However, in accordance with the invention, by some embodiments thereof, the heating device is embedded in or forms parts of objects having an entirely different purpose. For example, the heating element may form a structural component in a piece of furniture thus having a dual role in such an object. Thus, the term xe2x80x9cheating devicexe2x80x9d should be understood, depending on the context, as referring also to such dual-role objects.
The present invention allows in fact to design heating elements for every purpose, need and in any desired form. This unique feature of the invention allows to incorporate the heating element as a structural element or as an add-on element in a large variety of objects, including various domestic constructional units (e.g. door frames) furniture, etc.
The heating element of the invention dissipates heat at practically any desired power rating, with a working temperature of the element which is way below the working temperature of the heating element of prior art devices operating with a comparable power rating. For example, in a heating device of the invention suited for domestic interior heating, the 4V heating element may be designed to operate with a working temperature of 70-80xc2x0, which is the conventional output temperature of heating devices, and accordingly, the heating element may be placed and form the external, output surface of the heating device.
In order to avoid electric shock hazards, the heating element of the invention may be designed to operate under relatively low voltage, ranging, depending on the application, between 1V and 24V (which is the conventional upper limit in low voltage systems).
In accordance with the present invention, the heating element to suit a specific application is designed on the basis of novel developed relationships which allow to match the physical parameters of the heating element (dimensions and specific resistivity), to the electrical parameters of the heating element (desired voltage and power rating). These relations allow to choose the appropriate heating element to suit a specific application. Given the fact that the heating element operates at a relatively low temperature, it may be made from a wide variety of alloys, which cannot be used in conventional (prior art) heating devices, such as aluminum, stainless steel, copper, etc.
According to the general teaching of the invention, there is thus provided a heating device for heating an object to a required temperature, the heating device comprising a heating element and a power source for supplying voltage to the heating element, wherein:
the voltage U to be supplied to the heating element is selected in accordance with a predetermined application of the heating device;
the heating element is made of a selected electric conductive material of a specific resistivity xcfx81, is substantially flat, and has a substantially rectangular cross section, so as to define at least one major outer surface through which a given heating power required for heating said object to said required temperature is dissipated, length l, width b and thickness xcex4 of the heating element being selected such as to satisfy the following relations:       b    ≥          k      ·      I      ·                        ρ          δ                          l    ≥          k      ·      U      ·                        δ          ρ                    
xe2x80x83wherein I is the electric current passing through the heating element; xcfx81 is the specific resistivity (Ohmxc2x7mm2/m); xcex4 is the thickness of the heating element in millimeters; k is a correction coefficient providing the units of length and width in meters and millimeters, respectively, and selected such that an increase in the value of k results in an increase in the surface area of the heating element and decrease in the surface temperature of the heating element;
the heating device thereby providing a desired temperature gradient between said at least one major surface of the heating element and said required temperature.
Where the electric heating device is employed to heat air at an initial ambient temperature of about 20xc2x0 C. (room temperature) to a temperature of 100xc2x0 C. or more, or to heat a water based medium consisting of at least 50% water which is initially at room temperature, to a temperature up to about 50xc2x0 C., k will preferably be within the range of 0.2-0.6.
In case the heating device of the invention is employed to heat air at room temperature to a temperature of up to about 90xc2x0 C. or less, k will preferably be above 0.6.
The present invention allows to design a heating element to suit, practically, any desired need. At times, the length and width of the elements are predetermined by the shape of the heating device, leaving a certain degree of freedom in the choice of alloy (and hence of the specific resistivity xcfx81) and thickness of the element. This may be the case, for example, in heating devices in which the heating element is incorporated in another object, e.g. a piece of furniture. In other cases, the material and hence the specific resistivity xcfx81 is predetermined, leaving a degree of freedom for other physical parameters, being one of the length, width and thickness. This may be the case, for example, in heating devices where the heating element is intended to come into direct contact with a food item where the alloy from which the heating element is made will typically be stainless steel. These are only examples, but it is clear that it is possible, for practically all applications, to find, based on the above relations for length and width, a combination of parameters which allow to design an appropriate heating element, namely, a heating element which provides as low as desired temperature gradient between the outer surface of the heating element and the required temperature of the heated body.
By a specific aspect of the invention there is provided a heating device comprising a heating element incorporated as a structural element in a stationary object such as a piece of furniture, door or window frames, etc. In accordance with a further specific aspect of the invention, there is provided a heating device comprising a heating element embedded in or enclosed within such a stationary object.
Another specific aspect of the invention concerns a device for food heating. The term xe2x80x98food heatingxe2x80x9d should be understood as referring to one of a variety of heat-based food processing techniques including cooking, baking or grilling.
The novelty in the cooking device of the invention is in that it comprises a metal body which is either in contact with food or which is in proximity with food without any object between it and the food, said metal body serving as a heating element being thus connected to a power source for passing heating electric current therethrough.
In accordance with one embodiment of this latter aspect, there is provided a cooking device for heating of liquid food, comprising a metal vessel for holding the food having metal walls serving as heating elements, and comprising a power source for passing low voltage, high electric current therethrough, thereby heating the liquid food contained therein. The electrical current parameter useful for such an application is typically voltage ranging from 1.0V to 12V with a power rating of 1-2 KWatt.
In accordance with another embodiment of the latter aspect, there is provided a device for heating solid food items, in which the solid food items are placed in direct contact with a metal plate, said metal plate serving also as a heating element and being connected to an electric power source for passing a heating electric current therethrough.
In accordance with yet another embodiment of the latter aspect, there is provided a device for heating food by means of heat irradiation onto the food, comprising a food-containing enclosure having one or more metal walls, at least one of said metal walls serving as a heating element to heat said enclosure and being connected to a power source for passing a heating electric current therethrough.
By a further aspect, the present invention provides a heating device adapted to be worn or held on a body part for heating of that body part. By one embodiment of this aspect, such a device comprises a cloth or a cloth base matrix with a heating element embedded therein, the heating element having the above specifications. In accordance with another embodiment, the device comprises a liquid or gel-containing enclosure having pliable walls and containing therein a heating element, being a heating element having the above characteristics.
By yet another aspect of the invention, there is provided a method for designing a substantially flat heating element having a substantially rectangular cross-section for implementing into a heating device for a predetermined application consisting of heating an object to a required temperature, the method comprising the steps of:
(a) defining a power rating W in accordance with the predetermined application;
(b) defining a range of operating potential U of the heating element in accordance with the predetermined application;
(c) selecting physical parameters of the heating element, so as to dissipate therethrough a heating power substantially equal to said power rating W, when the operating potential U falls onto the heating element, thereby providing a desired temperature gradient between the temperature of an outer surface of the heating element through which power is dissipated and said required temperature, wherein the selecting of the physical parameters comprises:
selecting an electrically conductive material with a specific resistivity xcfx81 for fabricating therefrom said heating element, and selecting dimensions of the heating element, so as to satisfy the following       b    ≥          k      ·      I      ·                        ρ          δ                          l    ≥          k      ·      U      ·                        δ          ρ                    
xe2x80x83wherein I is the electric current passing through the heating element; xcfx81 is the specific resistivity (Ohmxc2x7mm2/m); l is the length of the heating element in meters; b is the width of the heating element in millimeters; xcex4 is the thickness of the heating element in millimeters; k is a correction coefficient providing the units of length and width in meters and millimeters, respectively, and selected such that an increase in the value of k results in an increase in the surface area of the heating element and a decrease in the surface temperature of the heating element.
It will be appreciated that the actual working temperature in a heating device depends on a variety of factors, including the nature of the heating device environment, the exact geometry, etc., and therefore it is not always possible to predict the working temperature entirely on theoretical considerations.
Owing to the fact that the voltage to be supplied to the heating element is a varying parameter selected in accordance with a specific application, a step-down transformer should be used in a heating device according to the invention. However, at the frequencies of the existing power networks (i.e., 50 Hz and 60 Hz), the weight and dimensions of such a transformer are significantly high, which sometimes makes the use of the transformer inefficient. The development of modem inverters practically allows for eliminating weight and dimensions related problem, since the weight and dimensions of a transformer are in inverse proportion to the working frequency thereof.
The heating band according to the invention, due to its task, occupies a significantly large area, and consequently, the induction C of its electric circuit will be significantly high. On the one hand, since the inductive resistance, Rc, is proportional to the frequency f (Rc=kf), this problem must be overcome. On the other hand, due to the substantially large area and relatively high electric currents (as compared to those flowing in conventional heating devices), the heating band may behave like an antenna that produces an electromagnetic field presenting noise for electronic equipment in its vicinity.
The present invention also provides the solution for the above problem. This is implemented by accommodating wires connecting the heating band to a power source in the following manner. One of the wires is connected directly to one of the outputs of the heating band. The other wire, on its way to the other output of the heating band, passes along the entire circuit of the band so as to be substantially symmetrical to the band along its longitudinal axis. This enables to sharply reduce both the inductive resistance and the electromagnetic field.
Another problem arises when the condition for obtaining low-gradient heating is associated with the need to work with substantially high electric currents, and consequently, substantially low voltages. Hence, the wire connecting the power source to the heating element has to be made with a substantially high cross section, which may affect the flexibility of the wire and thus may be undesirable in some applications (e.g., physiotherapy). This problem can be solved in the following manner. Since the weight and dimensions of a transformer operating with high frequencies are significantly low, an output transformer can be taken out of the inverter and mounted in a heating element. As a result, a substantially low electric current flows through the wires connecting the power source to the heating element, thereby providing desired flexibility of the electrically conductive wires.
The case may be such that the supply of 220V or 110V to the heating element is undesirable for industrial safety provisions. In this case, an intermediate transformer reducing the voltage to 12V will function inside the inverter, while a transformer reducing the voltage to 1V and less will function inside the heating element. By this, the problem associated with the need to operate with substantially high currents, and consequently, relatively low voltages, can be solved.